Technically one may need permission from their parents to say Kaddish while they are alive.

I've never heard this applied to Kaddish D'Rabbanan. It's b'chlal not connected to mourning.

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I believe that outside Lubavitch, one whose parents are living does not generally say Kaddish d'Rabbanan.

I would find this suprising. In Yeshivos, where there is often no Chiyuv, whichever Bochur is Chazan will say it after Sheyiboneh and before Aleinu. What would they do in that situation? Or by a Siyum Masechta?

My father grew up in a non-Lubavitch family, and we lived in a non-Lubavitch community. After my bar mitzvah, we started davening in a Chabad House that had opened in our community. I davened for the amud, and before Aleinu, I saw the Kaddish d'Rabbanan in the siddur, so I started saying it. I wasn't expecting the Aramaic, so I stumbled through it. Afterwards, my father told me that only yesomim say it, but when he was going to stop me, the shliach had told him that once I had started it was ok, because some people do say it regardless. My father joked that breaking my teeth over the words probably taught me a lesson.

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